Heavy hydrocarbons represent a huge natural source of the world's total potential reserves of oil. Present estimates place the quantity of heavy hydrocarbon reserves at several trillion barrels, more than 5 times the known amount of the conventional, i.e. non-heavy, hydrocarbon reserves. This is partly because heavy hydrocarbons are generally difficult to recover by conventional recovery processes and thus have not been exploited to the same extent as non-heavy hydrocarbons.
Heavy hydrocarbons also present many challenges topside after extraction from a formation. They possess very high viscosities which makes them difficult to pump in their native state. Additionally heavy hydrocarbons are characterised by high levels of unwanted compounds such as asphaltenes, trace metals and sulphur that need to be processed appropriately. Heavy hydrocarbons also contain ARN acids at ppm levels.
Another class of unwanted compound that is present in many hydrocarbons, and particularly in heavy hydrocarbons, is metal naphthenates. They may be present in crude hydrocarbon mixtures in significant amounts. For example, Doba crude oil has been reported to contain more than 400 ppm wt of metal naphthenates.
Metal naphthenates are often formed from naphthenic acids. Two main categories of naphthenic acids exist. These are: (1) naphthenic acids which are monoacids; and (2) ARN naphthenic acids, which are C80-82 tetracids. The ARN naphthenic acids are problematic during production because they form water soluble metal naphthenates that are sticky solids which harden on contact with air and cause fouling of pipelines and processing equipment. The present invention is concerned, however, with the naphthenic acids which are monoacids. These are problematic during production because they form oil soluble metal naphthenates which promote formation of stable emulsions.
Naphthenic acids of both categories are present in crude oil, under reservoir conditions, and reside in the hydrocarbon. Naphthenic acids which are monocarboxylic acids may be present in amounts of up to 12% wt. During extraction from a formation, depressurisation of the crude hydrocarbon mixture occurs as it moves up through the production tubing, and ultimately to the surface. This, in turn, causes CO2 present in the hydrocarbon mixture to flash and for the pH of the water present in the crude hydrocarbon mixture to increase. This results in the formation of naphthenate salts with ions from the water, e.g. calcium naphthenate and magnesium naphthenate. Some metal naphthenates may also form in the reservoir. This may occur, for example, if the pH of the water phase in the formation is relatively high, e.g. exceeds a pH of about 6.5 and the water has a relatively high salinity. The ARN naphthenic acids form water soluble metal naphthenates whereas the naphthenic acids which are monoacids form oil soluble metal naphthenates. The metal naphthenates present in a crude hydrocarbon mixture therefore derive from monocarboxylic naphthenic acids in the formation and/or produced from monocarboxylic naphthenic acids during hydrocarbon production from the formation.
Oil soluble metal naphthenates derived from monocarboxylic naphthenic acids are problematic during production of hydrocarbon from the formation because they cause significant problems during separation of crude hydrocarbon mixture from water. They tend to accumulate at the oil/water interface and act as surfactants. More specifically oil soluble metal naphthenates cause challenges including increased conductivity and poorer separation in the coalescer, formation of stable formations, water carryover, poor effluent water quality, scaling, corrosion and poisoning of refinery catalysts. Additionally the quality of fuel and coke derived from residue can, in some instances, be decreased when there are relatively high levels of calcium in the original oil phase.
In current commercially operated processes, the majority of the oil soluble metal naphthenate present in a crude hydrocarbon mixture extracted from a formation remains in the crude hydrocarbon mixture after the bulk separation process. Thus the crude hydrocarbon mixture transported to the refinery often contains significant amounts of oil soluble metal naphthenate and therefore metals such as calcium in the hydrocarbon phase. These must be removed during processing at the refinery in expensive processes. It is, in fact, estimated that the cost of handling the metal ions deriving from oil soluble metal naphthenates at the refinery is around 0.5 to 5 USD/bbl. The processes are also problematic. Problems have been experienced in the waste water treatment plant due to the increased levels of metal salts in the waste water and corrosion of overhead towers due to use of acetic acid to remove calcium naphthenate has been reported. Currently refineries are unable to deal with crude hydrocarbon comprising more than 100 ppm wt metal naphthenate.